Seismic geological surveys for underwater exploration, especially those in large water depths, is conducted by placing recorders on the seafloor as it provides the detector units to pick up on geological signals which would have been lost by transmission through the sea and/or which through shear waves would be rendered indistinguishable from background events. Additionally, the seafloor provides a stable environment, isolated from sounds, surface waves, temperature fluctuations and background noises which can interfere with the detection of geological events, allowing for high precision geological surveying. In the current state of technology ocean bottom sensor units for ocean bottom investigation are often positioned and retrieved individually, wherein each individual ocean bottom sensor unit is required to be fitted with an onboard power supply, clock, interrogation system, memory and positioning systems. Ocean bottom sensors aboard the ocean bottom sensor unit can be interrogated by an onboard interrogation unit which is powered by an onboard power source. It is common that onboard ocean bottom sensors are electrical and are, therefore, required to be powered by said onboard power source.
One of the disadvantages arising from individual deployment of ocean bottom sensor units and the use of electric sensors is that each unit is required to be fully self-sufficient, resulting in high equipment costs providing such self-sufficiency. Individual deployment further results in a high risk of loss of individual sensor units. Disadvantages concerning operation are possible discrepancies due to challenges in sufficiently precise time-synchronization between recorder units. Further disadvantages during deployment and retrieval is the requirement of accurate placement of each recorder node individually by remotely operated vehicles (ROVs) or autonomous underwater vehicles (AUVs) which is time consuming and costly.
Additionally, the use of electrical sensors involves high power consumption and a high susceptibility to electrical failure due to the presence of salt water conditions.
In other current systems multiple sensor units for ocean bottom sensing are joined by means of a rope or cable to enable rapid deployment and retrieval. Units in such systems may collectively share an energy source. However, these setups do not necessarily overcome the high operational energy demand as presented within self-sufficient sensor units. As energy losses occur through wiring and due to signal losses, sensor energy demand is often higher than would be the case when the sensor would be placed in closer proximity to a recorder device. Disadvantageously these setups many times require a direct surface side power supply due to the high energy need, or AUVs to provide such power. The systems with rope connections are limited in deployment depth due to difficulties in controlling deployment. Moreover, such systems are often susceptible to entanglement issues during deployment and retrieval. Hence, mechanical failure due to said entanglement issues and short circuiting due to additional electrical cabling are common. There is thusly a need for an ocean bottom sensing system which prevents the occurrence of entanglement issues and increases the deployability of its system, while overcoming the abovementioned disadvantages.